One of perfect binding machines which are well-known in the prior art is provided with a series of binding units and at least one clamp unit serially moving through the binding units, wherein a book block is held between a pair of clamp plates of the clamp unit and then, while the book block is conveyed by the clamp unit along the series of binding units, the perfect binding is performed (cf. for example, Patent Document 1).
FIG. 6 is a plan view schematically showing the configuration of the conventional perfect binding machine. Referring to FIG. 6, the perfect binding machine comprises a series of binding units (a milling unit B, a glue application unit C and a cover attachment unit D) which are arranged along a conveying path F, a single clamp unit 31 arranged for reciprocal movement along the conveying path F, and a drive mechanism (not shown) moving the clamp unit 31 along the conveying path F. The clamp unit 31 has stationary and movable clamp plates 31a, 31b, and a motor M1 for moving the movable clamp plate 31b in directions toward and away from the stationary clamp plate 31a. 
When the perfect binding is started, at a book block insertion position A, a book block (not shown) is inserted between the pair of clamp plates 31a, 31b of the clamp unit 31 and placed on an alignment plate 31c with its back face downwardly facing. Then the book block is held between the clamp plates 31a, 31b in such a way that back margins, which are located at both sides of the back face of the book block, protrudes downwardly from the clamp unit 31, and then conveyed by the clamp unit 31 toward the milling unit B along the conveying path F.
The milling unit B has a milling cutter 32a, a pair of stationary and movable guide plates 32b, 32c, and a motor M2 for moving the movable guide plates 32c in directions toward and away from the stationary guide plate 32b. While the book block moves through the milling cutter 32a, both sides of the book block protruding between the pair of clamp plates 31a, 31b passes between the pair of guide plates 32b, 32c. Thus the back face of the book block is cut while the book block is supported by the pair of guide plates 32b, 32c at the both sides thereof. After that, the book block is conveyed to the adhesive application unit C by the clamp unit 31.
The glue application unit C has a glue tank 33a filled with glue, a glue applying roller 33b, stationary and movable side glue applying rollers 33c, 33d, a motor M3 for moving the movable side glue applying roller 33d in directions toward and away from the stationary side glue applying roller 33c, and a roller 33e wiping off an excessive adhesive. When the application of glue is completed, the book block is conveyed to the cover attachment unit D by the clamp unit 31.
The cover attachment unit D includes a bottom plate 34c, stationary and movable nip plates 34a, 34b, and a motor M4 for moving the movable nip plate 34b in directions toward and away from the stationary nip plate 34a. 
A cover supply unit E includes a tray on which a cover Q is placed, and a cover feed mechanism conveying the cover Q from the tray onto the bottom plate 34c and the pair of nip plates 34a, 34b of the cover attachment unit D.
The cover feed mechanism has a pair of fold line forming rollers 35a, 35b for forming fold lines at predetermined positions on the cover Q. The fold line forming rollers 35a, 35b are composed of a stationary fold line forming roller 35a and a movable fold line forming roller 35b. The movable fold line forming roller 35b is moved by a motor M5 in directions toward and away from the stationary fold line forming roller 35a. 
The perfect binding machine includes a thickness measurement unit 36 for measuring a thickness of the book block. Prior to the start of the bookbinding operation of the machine, a thickness of the book block is measured by an operator through the use of the thickness measurement unit 36. Thus, a gap between the pair of clamp plates 31a, 31b, a gap between the pair of guide plates 32b, 32c of the milling unit B, a gap of the pair of side glue applying rollers 33c, 33d of the glue application unit C, a gap between the pair of nip plates 34a, 34b of the cover attachment unit D and a gap between the pair of fold line forming rollers 35a, 35b of the cover feed mechanism E are adjusted by a controller 37 based on the measurement value of the thickness of the book block.
Thus the book block is held between the pair of clamp plates 31a, 31b of the clamp unit 31 and then, while the book block is conveyed by the clamp unit 31 along the series of binding units B-D, the perfect binding is performed.
According to the glue application unit C of this perfect binding machine, prior to the start of the bookbinding operation of the machine, the gap between the pair of side glue application rollers 33c, 33d is adjusted such that the gap corresponds to the thickness of the book block to be bound. Therefore, if the back of the book block is undulated in its length direction although the book block is nipped between the pair of clamp plates 31a, 31b, the side glue application rollers 33c, 33d cannot engage with the back margins of the book block while being pushed against the back margins at a constant pressure throughout the length of the back margins during the passage of the book block over the glue application unit C, which causes a problem that the glue is unevenly applied to the back margins of the book block and the quality of the bookbinding is reduced.
Furthermore, in this glue application unit C, the gap between the pair of side glue application rollers 33c, 33d is adjusted by moving the movable side glue application roller 33d against the stationary side glue application roller 33c through the use of the motor M3, which complicates the gap adjustment mechanism, thereby raises the manufacturing cost of the glue application unit C and becomes a factor of growing the unit C in size.